Several million people in the U.S. suffer from overuse tendon injuries or tendinopathies, a common cause of pain and disability. One type, posterior tibial tendon dysfunction (PTTD), affects 3.3% of all women over the age of 40. While early signs manifest as tendonitis, the disease often progresses to severe flatfoot and can have a profound impact on ambulatory capacity. Because the severity and prognosis of PTTD are based on a subjective and imprecise physical exam, there is no way to predict which patients will do well with conservative bracing and therapy and which will ultimately require invasive surgery. This dilemma leads to prolonged unsuccessful conservative treatment in some patients and premature surgical intervention in others. Previous tendon studies strongly suggest that tendon disease and rehabilitation are closely associated with changes in the tendon's underlying composition and elasticity. Yet, there is no proven clinical tool to objectively and reliably measure tendon these properties in vivo. To address this limitation, we propose developing and testing ultrasound elasticity imaging (UEI) to quantify mechanical properties of the human PTT. Our objective is to gain an understanding of the relationship between tendon elasticity and the clinical severity of PTTD, the effect of tendon rehabilitation, and the prognosi for recovery. This project would 1) compare the mechanical properties of the PTT in patients with unilateral symptomatic flatfoot deformity with the contralateral control PTT, as well as that of healthy volunteers; 2) determine whether physical therapy induces mechanical changes in the PTT; and 3) determine whether UEI combined with discriminant functional analysis can predict the clinical outcome in subjects with advanced PTTD with the goal of determining group membership for new patients (conservative treatment or surgery). UEI has great potential as a portable, real-time, noninvasive modality to transform the current paradigm by providing a validated objective measure for evaluating tendinopathies, assisting with their prognosis, and prediction of clinical outcome from a particular therapy and its optimal timing. Success in this R21 project would justify a prospective clinical trial in a larger population to better define thresholds for predicting whether a particular patient should continue conservative treatment or opt for surgery. Such a prospective trial would ultimately determine the clinical usefulness of UEI in evidence based-management of PTTD (and possible other tendinopathies) and improving outcomes, including quality of life, in these patients.